Infrared detectors sense electromagnetic radiation having a wavelength, generally, between 0.5 and 15 .mu.m (or having an energy of 2 to 0.1 eV). Infrared detectors are used for a variety of purposes including target acquisition and tracking.
Infrared detectors have been incorporated into integrated circuits for mass production and miniaturization. Typically, they are fabricated in N.times.M arrays. Each element in the array is itself fabricated with a piece of semiconductor consisting of mercury, cadmium, and tellurium ("HgCdTe") which is operable to generate electron-hole pairs when struck by infrared radiation. The particular wavelength from which each element generates electron-hole pairs may be tuned by adjusting the ratio of mercury to cadmium in the semiconductor material.
In certain applications, it is desirable to be able to detect two distinct wavelengths of infrared radiation. The ratio of two different wavelengths of infrared radiation may be used by a targeting computer for target recognition and target distance calculations. Heretofore, two color infrared detectors have been manufactured using one of two device types. The first device type is a triple-layer-heterojunction diode. This structure employs two back-to-back diodes forming a three terminal semiconductor device. The second type of device is an N.times.M array consisting of twice the usual number of individual detector elements. Half of the elements are tuned to a first frequency while the second half are tuned to a second frequency. Both of these solutions have numerous disadvantages.
The triple-layer-heterojunction diode consists of two infrared sensitive N-type layers separated by a center P-type layer which is insensitive to infrared radiation. The three layers are aligned along a vertical axis which is generally parallel to the direction of propagation of the incident infrared radiation. A voltage is applied between the first and second and second and third layers such that each diode is reverse biased, either sequentially or simultaneously. If each N-type layer is tuned to a separate infrared wavelength, the detector can detect two infrared wavelengths. Unfortunately this structure requires the center P-type layer and its related interconnect structure. Also, this structure requires two separate sensing circuits to sense two distinct wavelengths. In the alternate, the device could use one sensing circuit and a multiplexer to alternately connect each N-type layer to the sensing circuit.
The second type of two color infrared detector simply uses twice as many elements to sense twice as many events. These elements are laid out in the same horizontal plane which is perpendicular to the axis of incident infrared radiation. This format requires geometrically more space which is oftentimes critical. In addition, this system is not a true two color detector since each set of array elements are not aligned along the same optical axis.
Therefore, a need has arisen for a two color infrared detector which is able to simultaneously detect two wavelengths of infrared light, which is compact, and which minimizes the complexity of the related sensing circuitry.